A PoE-capable switch port automatically supplies power to one of these connected devices if the switch senses that there is no power on the circuit:
a Cisco pre-standard powered device (such as a Cisco IP Phone or a Cisco Aironet Access Point)
an IEEE 802.3af-compliant powered device
an IEEE 802.3at-compliant powered device
A powered device can receive redundant power when it is connected to a PoE switch port and to an AC power source. The device does not receive redundant power when it is only connected to the PoE port.
After the switch detects a powered device, the switch determines the device power requirements and then grants or denies power to the device. The switch can also sense the real-time power consumption of the device by monitoring and policing the power usage.
The switch uses these protocols and standards to support PoE:
CDP with power consumption—The powered device notifies the switch of the amount of power it is consuming. The switch does not reply to the power-consumption messages. The switch can only supply power to or remove power from the PoE port.
Cisco intelligent power management—The powered device and the switch negotiate through power-negotiation CDP messages for an agreed-upon power-consumption level. The negotiation allows a high-power Cisco powered device, which consumes more than 7 W, to operate at its highest power mode. The powered device first boots up in low-power mode, consumes less than 7 W, and negotiates to obtain enough power to operate in high-power mode. The device changes to high-power mode only when it receives confirmation from the switch.
High-power devices can operate in low-power mode on switches that do not support power-negotiation CDP.
Cisco intelligent power management is backward-compatible with CDP with power consumption; the switch responds according to the CDP message that it receives. CDP is not supported on third-party powered devices; therefore, the switch uses the IEEE classification to determine the power usage of the device.
IEEE 802.3af—The major features of this standard are powered-device discovery, power administration, disconnect detection, and optional powered-device power classification. For more information, see the standard.
IEEE 802.3at—The PoE+ standard increases the maximum power that can be drawn by a powered device from 15.4 W per port to 30 W per port.
Powered-Device Detection and Initial Power Allocation
The switch detects a Cisco pre-standard or an IEEE-compliant powered device when the PoE-capable port is in the no-shutdown state, PoE is enabled (the default), and the connected device is not being powered by an AC adaptor.
After device detection, the switch determines the device power requirements based on its type:
A Cisco prestandard powered device does not provide its power requirement when the switch detects it, so a switch that does not support PoE+ allocates 15.4 W as the initial allocation for power budgeting; a PoE+ switch allocates 30 W (PoE+).
The initial power allocation is the maximum amount of power that a powered device requires. The switch initially allocates this amount of power when it detects and powers the powered device. As the switch receives CDP messages from the powered device and as the powered device negotiates power levels with the switch through CDP power-negotiation messages, the initial power allocation might be adjusted.
The switch classifies the detected IEEE device within a power consumption class. Based on the available power in the power budget, the switch determines if a port can be powered. Table 1 lists these levels.
Table 1 IEEE Power Classifications
Class
Maximum Power Level Required from the Switch
0 (class status unknown)
15.4 W
1
4 W
2
7 W
3
15.4 W
4
30 W PoE+ devices only
The switch monitors and tracks requests for power and grants power only when it is available. The switch tracks its power budget (the amount of power available on the switch for PoE). The switch performs power-accounting calculations when a port is granted or denied power to keep the power budget up to date.
After power is applied to the port, the switch uses CDP to determine the CDP-specific power consumption requirement of the connected Cisco powered devices, which is the amount of power to allocate based on the CDP messages. The switch adjusts the power budget accordingly. This does not apply to third-party PoE devices. The switch processes a request and either grants or denies power. If the request is granted, the switch updates the power budget. If the request is denied, the switch ensures that power to the port is turned off, generates a syslog message, and updates the LEDs. Powered devices can also negotiate with the switch for more power.
With PoE+, powered devices use IEEE 802.3at and LLDP power with media dependent interface (MDI) type, length, and value descriptions (TLVs), Power-via-MDA TLVs, for negotiating power up to 30 W. Cisco pre-standard devices and Cisco IEEE powered devices can use CDP or the IEEE 802.3at power-via-MDI power negotiation mechanism to request power levels up to 30 W.
Note
The initial allocation for Class 0, Class 3, and Class 4 powered devices is 15.4 W. When a device starts up and uses CDP or LLDP to send a request for more than 15.4 W, it can be allocated up to the maximum of 30 W.
Note
The CDP-specific power consumption requirement is referred to as the actual power consumption requirement in the software configuration guides and command references.
If the switch detects a fault caused by an undervoltage, overvoltage, overtemperature, oscillator-fault, or short-circuit condition, it turns off power to the port, generates a syslog message, and updates the power budget and LEDs.
The PoE feature operates the same whether or not the switch is a stack member. The power budget is per-switch and independent of any other switch in the stack. Election of a new active switch does not affect PoE operation. The active switch keeps track of the PoE status for all switches and ports in the stack and includes the status in output displays.
The stacking-capable switch also supports StackPower, which allows the power supplies to share the load across multiple systems in a stack when you connect the switches with power stack cables. You can manage the power supplies of up to four stack members as a one large power supply.
Power Management Modes
The switch supports these PoE modes:
auto—The switch automatically detects if the connected
device requires power. If the switch discovers a powered device connected to the port
and if the switch has enough power, it grants power, updates the power budget, turns
on power to the port on a first-come, first-served basis, and updates the LEDs. For
LED information, see the hardware installation guide.
If the switch has enough power for all the powered devices, they all come up. If
enough power is available for all powered devices connected to the switch, power is
turned on to all devices. If there is not enough available PoE, or if a device is
disconnected and reconnected while other devices are waiting for power, it cannot be
determined which devices are granted or are denied power.
If granting power would exceed the system power budget, the switch denies power,
ensures that power to the port is turned off, generates a syslog message, and updates
the LEDs. After power has been denied, the switch periodically rechecks the power
budget and continues to attempt to grant the request for power.
If a device being powered by the switch is then connected to wall power, the switch
might continue to power the device. The switch might continue to report that it is
still powering the device whether the device is being powered by the switch or
receiving power from an AC power source.
If a powered device is removed, the switch automatically detects the disconnect and
removes power from the port. You can connect a nonpowered device without damaging
it.
You can specify the maximum wattage that is allowed on the port. If the IEEE class
maximum wattage of the powered device is greater than the configured maximum value,
the switch does not provide power to the port. If the switch powers a powered device,
but the powered device later requests through CDP messages more than the configured
maximum value, the switch removes power to the port. The power that was allocated to
the powered device is reclaimed into the global power budget. If you do not specify a
wattage, the switch delivers the maximum value. Use the auto setting on any
PoE port. The auto mode is the default setting.
static—The switch pre-allocates power to the port (even
when no powered device is connected) and guarantees that power will be available for
the port. The switch allocates the port configured maximum wattage, and the amount is
never adjusted through the IEEE class or by CDP messages from the powered device.
Because power is pre-allocated, any powered device that uses less than or equal to
the maximum wattage is guaranteed to be powered when it is connected to the static
port. The port no longer participates in the first-come, first-served model.
However, if the powered-device IEEE class is greater than the maximum wattage, the
switch does not supply power to it. If the switch learns through CDP messages that
the powered device needs more than the maximum wattage, the switch shuts down the
powered device.
If you do not specify a wattage, the switch pre-allocates the maximum value. The
switch powers the port only if it discovers a powered device. Use the
static setting on a high-priority interface.
never—The switch disables powered-device detection and
never powers the PoE port even if an unpowered device is connected. Use this mode
only when you want to make sure that power is never applied to a PoE-capable port,
making the port a data-only port.
When policing of the real-time power consumption is enabled, the switch takes action when a
powered device consumes more power than the maximum amount allocated, also referred to as
the cutoff-power value.
When PoE is enabled, the switch senses the real-time power consumption of the powered
device. The switch monitors the real-time power consumption of the connected powered
device; this is called power monitoring or power sensing. The switch also
polices the power usage with the power policing feature.
Power monitoring is backward-compatible with Cisco intelligent power management and
CDP-based power consumption. It works with these features to ensure that the PoE port can
supply power to the powered device.
The switch senses the real-time power consumption of the connected device as follows:
The switch monitors the real-time power consumption on individual ports.
The switch records the power consumption, including peak power usage. The switch
reports the information through the CISCO-POWER-ETHERNET-EXT-MIB.
If power policing is enabled, the switch polices power usage by comparing the
real-time power consumption to the maximum power allocated to the device. The maximum
power consumption is also referred to as the cutoff power on a PoE port.
If the device uses more than the maximum power allocation on the port, the switch can
either turn off power to the port, or the switch can generate a syslog message and
update the LEDs (the port LED is now blinking amber) while still providing power to
the device based on the switch configuration. By default, power-usage policing is
disabled on all PoE ports.
If error recovery from the PoE error-disabled state is enabled, the switch
automatically takes the PoE port out of the error-disabled state after the specified
amount of time.
If error recovery is disabled, you can manually re-enable the PoE port by using the
shutdown and no shutdown
interface configuration commands.
If policing is disabled, no action occurs when the powered device consumes more than
the maximum power allocation on the PoE port, which could adversely affect the
switch.
Power Consumption Values
You can configure the initial power allocation and the maximum power allocation on a port.
However, these values are only the configured values that determine when the switch should
turn on or turn off power on the PoE port. The maximum power allocation is not the same as
the actual power consumption of the powered device. The actual cutoff power value that the
switch uses for power policing is not equal to the configured power value.
When power policing is enabled, the switch polices the power usage at the switch
port, which is greater than the power consumption of the device. When you are
manually set the maximum power allocation, you must consider the power loss over the cable
from the switch port to the powered device. The cutoff power is the sum of the rated power
consumption of the powered device and the worst-case power loss over the cable.
The actual amount of power consumed by a powered device on a PoE port is the cutoff-power
value plus a calibration factor of 500 mW (0.5 W). The actual cutoff value is approximate
and varies from the configured value by a percentage of the configured value. For example,
if the configured cutoff power is 12 W, the actual cutoff-value is 11.4 W, which is 0.05%
less than the configured value.
We recommend that you enable power policing when PoE is enabled on your switch. For
example, if policing is disabled and you set the cutoff-power value by using the
power inline auto max6300 interface configuration command, the configured maximum power
allocation on the PoE port is 6.3 W (6300 mW). The switch provides power to the connected
devices on the port if the device needs up to 6.3 W. If the CDP-power negotiated value or
the IEEE classification value exceeds the configured cutoff value, the switch does not
provide power to the connected device. After the switch turns on power on the PoE port, the
switch does not police the real-time power consumption of the device, and the device can
consume more power than the maximum allocated amount, which could adversely affect the
switch and the devices connected to the other PoE ports.
If a power supply is removed and replaced by a new power supply with less power and
the switch does not have enough power for the powered devices, the switch denies
power to the PoE ports in auto mode in descending order of the port numbers. If the
switch still does not have enough power, the switch then denies power to the PoE
ports in static mode in descending order of the port numbers.
If the new power supply supports more power than the previous one and the switch now
has more power available, the switch grants power to the PoE ports in static mode in
ascending order of the port numbers. If it still has power available, the switch then
grants power to the PoE ports in auto mode in ascending order of the port
numbers.
For most situations, the default configuration (auto mode) works well, providing
plug-and-play operation. No further configuration is required. However, perform this task to configure a PoE port for a higher priority, to make it data only, or to
specify a maximum wattage to disallow high-power powered devices on a port.
When you make PoE configuration changes, the port being configured drops power.
Depending on the new configuration, the state of the other PoE ports, and the state of
the power budget, the port might not be powered up again. For example, port 1 is in the
auto and on state, and you configure it for static mode. The switch removes power from
port 1, detects the powered device, and repowers the port. If port 1 is in the auto and
on state and you configure it with a maximum wattage of 10 W, the switch removes power
from the port and then redetects the powered device. The switch repowers the port only
if the powered device is a class 1, class 2, or a Cisco-only powered device.
SUMMARY STEPS
1.configure terminal
2.interfaceinterface-id
3.power inline {auto [maxmax-wattage] | never | static [maxmax-wattage]}
4.end
5.show power inline
[interface-id | module switch-number]
DETAILED STEPS
Command or Action
Purpose
Step 1
configure terminal
Example:
Switch# configure terminal
Enters global configuration mode.
Step 2
interfaceinterface-id
Example:
Switch(config)# interface gigabitethernet2/0/1
Specifies the physical port to be configured, and enters interface configuration
mode.
Step 3
power inline {auto [maxmax-wattage] | never | static [maxmax-wattage]}
Example:
Switch(config-if)# power inline auto
Configures the PoE mode on the port. The keywords have these meanings:
auto—Enables powered-device detection. If enough power is available,
automatically allocates power to the PoE port after device detection. This
is the default setting.
never—Disables device detection, and disable power to the port.
Note
If a port has a Cisco powered device connected to it, do not use the power
inline never command to configure the port. A false link-up can occur,
placing the port into the error-disabled state.
static—Enables powered-device detection. Pre-allocate (reserve) power
for a port before the switch discovers the powered device. The switch
reserves power for this port even when no device is connected and guarantees
that power will be provided upon device detection.
The switch allocates power to a port configured in static mode before it allocates
power to a port configured in auto mode.
Step 4
end
Example:
Switch(config-if)# end
Returns to privileged EXEC mode.
Step 5
show power inline
[interface-id | module switch-number]
Example:
Switch# show power inline
Displays PoE status for a switch or a switch stack, for the specified interface,
or for a specified stack member.
The moduleswitch-number keywords are supported only on stacking-capable switches.
Budgeting Power for Devices Connected to a PoE Port
When Cisco powered devices are connected to PoE ports, the switch uses Cisco Discovery Protocol (CDP) to determine the CDP-specific power consumption of the devices, and the switch adjusts the power budget accordingly. This does not apply to IEEE third-party powered devices. For these devices, when the switch grants a power request, the switch adjusts the power budget according to the powered-device IEEE classification. If the powered device is a class 0 (class status unknown) or a class 3, the switch budgets 15,400 mW for the device, regardless of the CDP-specific amount of power needed. If the powered device reports a higher class than its CDP-specific consumption or does not support power classification (defaults to class 0), the switch can power fewer devices because it uses the IEEE class information to track the global power budget.
By using the power inline consumption wattage
interface configuration command or the power inline consumption default wattage
global configuration command, you can override the default power requirement specified by the IEEE classification. The difference between what is mandated by the IEEE classification and what is actually needed by the device is reclaimed into the global power budget for use by additional devices. You can then extend the switch power budget and use it more effectively.
Caution
You should carefully plan your switch power budget, enable the power monitoring feature, and make certain not to oversubscribe the power supply.
Note
When you manually configure the power budget, you must also consider the power loss over the cable between the switch and the powered device.
Perform this task to configure the amount of power
budgeted to a powered device connected to each PoE port on a switch:
SUMMARY STEPS
1.configure terminal
2.no cdp run
3.power inline consumption defaultwattage
4.end
5.show power inline consumption default
DETAILED STEPS
Command or Action
Purpose
Step 1
configure terminal
Example:
Switch# configure terminal
Enters global configuration mode.
Step 2
no cdp run
Example:
Switch(config)# no cdp run
(Optional) Disables CDP.
Step 3
power inline consumption defaultwattage
Example:
Switch(config)# power inline consumption default 5000
Configures the power consumption of powered devices connected to each the PoE port
on the switch.
The range for each device is 4000 to 30000 mW (PoE+). The default is 30000 mW.
Note
When you use this command, we recommend you also enable power policing.
Step 4
end
Example:
Switch(config)# end
Returns to privileged EXEC mode.
Step 5
show power inline consumption default
Example:
Switch# show power inline consumption default
Displays the power consumption status.
Budgeting Power to a Specific PoE Port
Perform this task to configure the amount of power
budgeted to a powered device connected to a specific PoE port:
SUMMARY STEPS
1.configure terminal
2.no cdp run
3.interfaceinterface-id
4.power inline consumptionwattage
5.end
6.show power inline consumption
DETAILED STEPS
Command or Action
Purpose
Step 1
configure terminal
Example:
Switch# configure terminal
Enters global configuration mode.
Step 2
no cdp run
Example:
Switch(config)# no cdp run
(Optional) Disables CDP.
Step 3
interfaceinterface-id
Example:
Switch(config)# interface gigabitethernet2/0/1
Specifies the physical port to be configured, and enter interface configuration
mode.
Step 4
power inline consumptionwattage
Example:
Switch(config-if)# power inline consumption 5000
Configures the power consumption of a powered device connected
to a PoE port on the switch.
The range for each device is 4000 to 30000 mW (PoE+). The default is 30000 mW.
Step 5
end
Example:
Switch(config-if)# end
Returns to privileged EXEC mode.
Step 6
show power inline consumption
Example:
Switch# show power inline consumption
Displays the power consumption data.
Configuring Power Policing
By default, the switch monitors the real-time power consumption of connected powered
devices. You can configure the switch to police the power usage. By default, policing is
disabled.
SUMMARY STEPS
1.configure terminal
2.interfaceinterface-id
3.power inline police [action{log | errdisable}]
4.exit
5.Use one of the following:
errdisable detect cause inline-power
errdisable recovery cause inline-power
errdisable recovery interval interval
6.exit
7.Use one of the following:
show power inline police
show errdisable recovery
DETAILED STEPS
Command or Action
Purpose
Step 1
configure terminal
Example:
Switch# configure terminal
Enters global configuration mode.
Step 2
interfaceinterface-id
Example:
Switch(config)# interface gigabitethernet2/0/1
Specifies the physical port to be configured, and enter interface configuration
mode.
Step 3
power inline police [action{log | errdisable}]
Example:
Switch(config-if)# power inline police
If the real-time power consumption exceeds the maximum power allocation on the
port, configures the switch to take one of these actions:
power inline police—Shuts down the PoE port, turns off power to it,
and puts it in the error-disabled state.
Note
You can enable error detection for the PoE error-disabled cause by using the
errdisable detect cause inline-power global configuration command.
You can also enable the timer to recover from the PoE error-disabled state by
using the errdisable recovery cause inline-powerintervalinterval global configuration command.
power inline police action errdisable—Turns off power to the port if the real-time power consumption exceeds the maximum power allocation on the port.
power inline police action log—Generates a syslog message while still providing power to
the port.
If you do not enter the action log keywords, the default action shuts down
the port and puts the port in the error-disabled state.
Step 4
exit
Example:
Switch(config-if)# exit
Returns to global configuration mode.
Step 5
Use one of the following:
errdisable detect cause inline-power
errdisable recovery cause inline-power
errdisable recovery interval interval
Example:
Switch(config)# errdisable detect cause inline-power
Switch(config)# errdisable recovery cause inline-power
Switch(config)# errdisable recovery interval 100
(Optional) Enables error recovery from the PoE error-disabled state, and
configures the PoE recover mechanism variables.
By default, the recovery interval is 300 seconds.
For
interval interval, specifies the time in seconds to recover from the
error-disabled state. The range is 30 to 86400.
Step 6
exit
Example:
Switch(config)# exit
Returns to privileged EXEC mode.
Step 7
Use one of the following:
show power inline police
show errdisable recovery
Example:
Switch# show power inline police
Switch# show errdisable recovery
Displays the power monitoring status, and verify the error recovery settings.
Monitoring Power Status
Table 2 Show Commands for Interfaces
Command
Purpose
show env power switch [switch-number]
(Optional) Displays the status of the internal power supplies for each
switch in the stack or for the specified switch. The range is 1 to 9,
depending on the switch member numbers in the stack.
These keywords are available only on stacking-capable switches.
show power inline [interface-id | module switch-number]
Displays PoE status for a switch or switch stack, for an interface, or for a
specific switch in the stack.
[no] power inline consumption default wattage global configuration command
[no] power inline consumption wattage
interface configuration command
this caution message appears:
%CAUTION: Interface Gi1/0/1: Misconfiguring the 'power inline consumption/allocation' command may cause damage to the
switch and void your warranty. Take precaution not to oversubscribe the power supply. It is recommended to enable power
policing if the switch supports it. Refer to documentation.